It was standing room only in Rohnert Park City Hall’s council chambers last Wednesday night May 21, when the Sonoma County Water Agency (SCWA) held another of its community forums. The next and final one is due in Santa Rosa next week.

The SCWA pointed out key messages right up front: “On the average, groundwater pumping has caused an imbalance, this imbalance can lower groundwater levels, reduce stream flows and affect ecosystems, pro-active development of a Groundwater Management Plan is in process and its success relies upon well owners and stakeholder participation.”

Proper impetus for this meeting is the persistent drought gripping the county this year. People wanted answers to the main question. That question is what affect will the calculated loss of 3,300 acres of groundwater by SCWA estimates during this drought year have on water supplies to citizenry? (Background on the underground water supply issue can be found in last week’s Coffee Grounds column).

The SCWA’s “concerned” about this loss figure. It’s figuring out ways to shrink the 3,300 estimate and the best method is achieving public participation via strict water-saving measures in individual homes, multi-family complexes, farms and ranches. Especially the latter two since agricultural use takes up 84 percent of the 261-square miles of the groundwater basin surveyed. This percentage includes livestock, vineyards, hayfields, pastures, dairies, vegetable farms, etc.

Survey by USGS computers

It’s interesting to note the United States Geological Service (USGS) seven-year survey of the Santa Rosa Plain was very 21st century in that it was all done by computer. No deep-sea divers with air hoses to the surface and no scuba divers with oxygen tanks on their backs prowling underwater. The USGS called their computer work “a model…a highly idealized approximation of the actual system.”

The model consisted of 16,741 Hydrological Response Units (HRU), each 10 acres in size, laid out by computer on the Santa Rosa Plain. A custom software program tapped each HRU to collect the data on streams, underground topography formations and depth of water. There were no instrument sticks sunk above ground in each HRU, the computer did it all. It took years for the USGS to amass all the data from each HRU.

Two water basins in SR Plain

What they discovered was the underground water was actually two basins, the Windsor Basin in the north and the Cotati Basin to the south, separated by a rocky protuberance 1,000 feet underwater called the Trenton Ridge. Water depth in both basins was variable, with Windsor’s about 4,500 feet and part of Cotati’s was as deep as 10,000 feet. The Russian River was not included in the study because it’s out of the Santa Rosa Plain and off the northwest corner of the plain.

The Laguna de Santa Rosa runs south to north near the western edge of the Plain where it connects to the Russian River. The Santa Rosa Plain is bisected in the center by the Rodgers Creek Earthquake Fault, located in the hills east of Rohnert Park, across the eastern suburbs of Santa Rosa and on up to Mendocino County.

Two county supervisors spoke

Marcus Trotta, hydrologist with the SCWA and project manager, along with Tim Parker of the Parker Groundwater Co., presented a lengthy PowerPoint survey of the project. Facilitator was Rich Wilson of the Center for Collaborative Policy. Two Sonoma County supervisors, David Rabbitt and Shirlee Zane, both spoke of the importance of the plan.

Also present were RP City Councilman Jake Mackenzie, RP Public Works Director John MacArthur, Cotati’s Public Works Director Damien O’Bid, Cotati Mayor John Dell’Osso and six members of the Santa Rosa Plain Advisory Panel (SRPAP), a 30-member group of stakeholders working with SCWA and other agencies. No one spoke for the Graton Resort and Casino, although they are members of SRPAP.